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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Tue, 5 Jul 2016 14:47:45 +0200
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Subject: Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the
 hot-path
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To: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, "Emilio G. Cota" <cota@braap.org>
Cc: mttcg@listserver.greensocs.com, peter.maydell@linaro.org, claudio.fontana@huawei.com, Peter Crosthwaite <crosthwaite.peter@gmail.com>, jan.kiszka@siemens.com, mark.burton@greensocs.com, a.rigo@virtualopensystems.com, qemu-devel@nongnu.org, serge.fdrv@gmail.com, bobby.prani@gmail.com, rth@twiddle.net, fred.konrad@greensocs.com



On 05/07/2016 13:14, Alex Bennée wrote:
> /*
>  * Patch the last TB with a jump to the current TB.
>  *
>  * Modification of the TB has to be protected with tb_lock which can
>  * either be already held or taken here.
>  */
> static inline void maybe_patch_last_tb(CPUState *cpu,
>                                        TranslationBlock *tb,
>                                        TranslationBlock **last_tb,
>                                        int tb_exit,
>                                        bool locked)
> {
>     if (cpu->tb_flushed) {
>         /* Ensure that no TB jump will be modified as the
>          * translation buffer has been flushed.
>          */
>         *last_tb = NULL;
>         cpu->tb_flushed = false;
>     }
> #ifndef CONFIG_USER_ONLY
>     /* We don't take care of direct jumps when address mapping changes in
>      * system emulation. So it's not safe to make a direct jump to a TB
>      * spanning two pages because the mapping for the second page can change.
>      */
>     if (tb->page_addr[1] != -1) {
>         *last_tb = NULL;
>     }
> #endif
>     /* See if we can patch the calling TB. */
>     if (*last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
>         if (!locked) {
>             tb_lock();
>         }
>         tb_add_jump(*last_tb, tb_exit, tb);
>         if (!locked) {
>             tb_unlock();
>         }
>     }
> }

Why not add tb_lock_recursive() and tb_lock_reset()?

Paolo